Manufacturing process for striae-free multicomponent chalcogenide glasses via multiple fining steps

ABSTRACT

The present invention provides for synthesizing high optical quality multicomponent chalcogenide glasses without refractive index perturbations due to striae, phase separation or crystal formation using a two-zone furnace and multiple fining steps. The top and bottom zones are initially heated to the same temperature, and then a temperature gradient is created between the top zone and the bottom zone. The fining and cooling phase is divided into multiple steps with multiple temperature holds.

PRIORITY CLAIM

The present application is a divisional application of U.S. applicationSer. No. 15/059,637, filed on Mar. 3, 2016 by Vinh Q. Nguyen et al.,entitled “MANUFACTURING PROCESS FOR STRIAE-FREE MULTICOMPONENTCHALCOGENIDE GLASSES VIA MULTIPLE FINING STEPS,” which was anon-provisional application claiming the benefit of U.S. ProvisionalApplication No. 62/127,305, filed on Mar. 3, 2015 by Vinh Q. Nguyen etal., entitled “MANUFACTURING PROCESS FOR STRIAE-FREE MULTICOMPONENTCHALCOGENIDE GLASSES VIA MULTIPLE FINING STEPS,” the entire contents ofboth are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates making striae-free multicomponentchalcogenide glasses with uniform refractive index.

Description of the Prior Art

Chalcogenide glasses comprise at least one chalcogen element (S, Se orTe) and other elements including, but not limited to, Ge, As, Ga, Sn, Sband transmit infrared light (IR) from between about 1 μm to about 12 μmor greater, depending on composition. The infrared transmittingchalcogenide glasses and optical fibers encompass the IR region ofinterest with numerous applications including thermal imaging,temperature monitoring, and medical applications. Also, chalcogenideglass fibers may be developed for IR missile warning systems and laserthreat warning systems to provide superior aircraft survivability, andhigh energy IR power delivery using for example, but not limited to, CO(5.4 μm) and CO₂ (10.6 μm) lasers (Sanghera et al., “IR fiber opticsdevelopment at the Naval Research Laboratory,” SPIE, 3950, 180-185(2000) and Sanghera et al., “Applications of Chalcogenide Glass OpticalFibers at NRL,” J. Optoelectronics and Advanced Materials 3 (3), 627-460(2001)). In addition, these fibers may be developed for remote fiberoptic chemical sensor systems for military facility clean-up and otherindustrial applications. High quality infrared transmitting opticalfibers enable applications in remote chemical sensors to detectcontaminants in groundwater, environmental pollution monitoring, Ramanamplifiers, optical ultra-fast switches for telecommunications, fibersources in the infrared for sensors, biomedical surgery and tissuediagnostics, and other civil/industrial process monitoring applications.Chalcogenide glasses may also be used as bulk optical elements,including windows, lenses, prisms, beam splitters and the like, and musthave high compositional uniformity and homogeneity in order to maintainaccurate control of light rays passing through the glass and to achievesatisfactory optical results.

Chalcogenide glasses based on arsenic and sulfur may be developed foruse in many defense applications including high energy IR laser powerdelivery for infrared countermeasures and chemical sensors for facilityclean up. The properties of the chalcogenide-based glasses, includingoptical, physical and thermal properties, such as refractive index,dispersion, thermo-optic coefficient, glass transition temperature,viscosity profile, hardness, fracture toughness, thermal expansion,density, nonlinear index, fluorescence and others, can be tailoredthrough composition. However, some chalcogenide glass compositions withtechnologically useful properties may be thermodynamically unstablewhereby crystallites or other inhomogeneities, including phase-separatedglassy regions or devitrified regions, form within the glass duringsynthesis, melting or processing. When synthesized using the methods ofprior art, this thermodynamic instability limits the physical size ofthe glass that may be fabricated (such as Ge₃₀ As₂₂ Se₂₃ Te₂₅), and insome cases optical quality glass may not be made in any size due tocrystal formation (such as Ge₁₃ As₃₂ Se₂₅ Te₃₀) (Kokorina, Glasses forInfrared Optics, CRC Press, Inc. (1996)). It is well-known in the art ofglass making that thermodynamically unstable glasses can be synthesizedby rapidly cooling the melt, but the glasses are not optical quality dueto striations that form upon rapid cooling.

The prior art methods to synthesize a chalcogenide glass from a melt aredemonstrated here by example.

EXAMPLE 1 Prior Art Process to Make Ge_(x)As_(y)S_((100-x-y-z))Se_(z)Glasses

First, germanium, arsenic, sulfur, and selenium precursors sufficient toconstitute a glass with the composition of x % at Ge, y % at As, z % atSe, and (100-x-y-z) % at S, where (0≤x≤10, 0≤z≤10 and 30≤x+y≤45) areloaded in a silica ampoule under an inert (e.g. Ar or nitrogen gas)atmosphere. As shown in FIG. 1, the ampoule 20 is cylindrical in shapeand has an axis 21 and a diameter 22. The ampoule 20 is connected to avacuum pump and evacuated for 4 hours at about 1×10⁻⁵ Torr. The ampoule20 shown in this example has a length, parallel to its axis 21, that islonger than the diameter 22, which is perpendicular to the axis 21, butthe ampoule 20 is not limited to this geometry and may have a diametergreater than its length, as is useful for casting large diameter glassfor large optics. The ampoule 20 containingGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass melt precursors 23 is sealedusing a methane (or hydrogen)/oxygen torch and placed inside a rockingfurnace 30 with a ±45° angle of inclination 11 where it is heated androcked according to a glass melting schedule, an example of which isshown in Table 1 (Sanghera et al., “Development of Low-Loss IRTransmitting Chalcogenide Glass Fibers,” SPIE vol. 2396, 71-77 (1995)).In Step 1, the top and bottom zones of the furnace are heated at a rateof 3° C./min from 20° C. (room temperature) to 750° C. The furnace thenremains at 750° C. for 10 hours and is actively rocked at an inclinationangle of ±45° to facilitate mixing and homogenization of the elementalcomponents.

In Step 2, the furnace motion is stopped and the furnace is set to avertical position (90° fixed angle, such that the axis of the ampoule isvertically aligned) and held at temperature (750° C.) for 1 hour tofacilitate fining and settling of the glass melt. At this stage, theampoule is also set up in the 90° vertical position parallel to thefurnace (FIG. 2).

In Step 3, the temperatures of both zones are reduced at a rate of 5°C./min to 440° C. and the temperature is held at 440° C. for 2 hrs. Asshown in FIG. 2, temperatures are measured at various points 301, 302,303, 304, 305 along the length of the ampoule 20 containingGe_(x)As_(y)S_((100-x-y-z)) Se_(z) glass melt 23 immediately prior toglass quenching. In this example, the heater 31 in the top zone 33 andthe heater 32 in the bottom zone 34 are set to the same temperature(440° C.), and beads of condensed glass 24 are seen to form at the topof the ampoule 20. The measured temperatures were as follows: T₃₀₁=430°C., T₃₀₂=435° C., T₃₀₃=438° C., T₃₀₄=441° C., and T₃₀₅=442° C.

In Step 4, the hot ampoule is removed from the furnace and submerged ina room temperature water bath for 30 seconds to quench the glass, and isthen placed in another furnace at 196° C. for 10 hours to anneal thesolid glass followed by slow cooling to room temperature at 1.0 C./min.

TABLE 1 Example of a prior art glass melting schedule forGe_(x)As_(y)S_((100−x−y−z))Se_(z) glass compositions in a two-zonefurnace. Heating Rate Temperature (° C.) Temperature (° C.) Dwell Step(° C./min) Top Zone Bottom Zone (hours) Furnace Position 1  3 750 750 10Rocking at ±45° inclination 2 — 750 750 1 Vertical 90° fixed. Fining. 3−5 440 440 2 Vertical 90° fixed. Fining. 4 Water quench

In Step 3 of the prior art process, although the top and bottom zones ofthe furnace are both set at the same temperature (440° C. in theexample) the actual measured temperature along the length of the ampoulecontaining the glass melt may vary. A temperature gradient (ΔT) of 12°C. was measured in the example and is due largely to convection heatloss through the top of the furnace. FIG. 3A shows a diagram of thermalconvection current 25 in the Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glassmelt 23 and glass condensation drops 24 on top of the cooler ampoule 20inside the furnace 30. FIG. 3B shows a photo of an ampoule 40 afterquenching with glass condensation 42 above the solid glass 41. Theeffect of convection heat loss causes thermal convection currents withinthe bulk glass, resulting in the condensation of glass beads above themelt at the cooler section of the ampoule which then drip back into themelt. These condensation beads may have a different composition than therest of the glass melt and this continual mass fluxing cycle can cause acompositional non-uniformity throughout the entire melt. Furthermore, asthe glass cools during Step 3, the composition of the glass near thesurface is changing as condensation of gaseous components (e.g. sulfur)from the closed system settle on the surface of the glass melt. Thermalconvection currents within the glass are present during cooling andallow this surface glass, with a slightly different composition, tobecome reincorporated into the bulk glass. The convection currents orswirls are not sufficient to thoroughly distribute or homogenize theglass, resulting in compositional gradients within the glass.

During water quenching of Step 4, the viscosity of the glass increasesas the glass melt cools and the compositional gradients become frozenresulting in striae in the bulk glass. Consequently, there arerefractive index perturbations in the striae-containing glass thatdegrade the optical quality of the glass and fiber made from this glass.FIG. 4A shows an IR-image of a hot steel grating viewed through a 1 inchdiameter, 4.0 inches thick cylinder (both faces polished) ofGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass of this example and reveals thepresence of striae and refractive index perturbations within the glass.FIG. 4B show an IR-image of a human hand and fingers viewed through a 1inch diameter, 4.0 inches thick cylinder (both faces polished) ofGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass of this example and reveals thepresence of striae and refractive index perturbations within the glass.

EXAMPLE 2 Prior Art Process to Make Striae-Free As₃₉S₆₁ Glasses

Nguyen et al., in another invention, teach a method to synthesizestriae-free arsenic sulfide-based chalcogenide glass (As₃₉ S₆₁) andother chalcogenide glasses (Nguyen et al., US 2015/0344342 (Dec. 3,2015)). In that invention, a furnace with an upper zone and a lower zoneis used for rocking and fining of the glass melt and the temperature ofthe upper zone is hotter than the lower zone by 100° C. during all stepsof the melting schedule, shown here in Table 2. The main feature of theprior art method employed by Nguyen et al. is that the temperature ofthe top zone in the fining and cooling steps (steps 3 & 4) is set to ahigher temperature than the bottom zone by 100° C., which has twobenefits: 1) convection currents within the glass melt are reduced and2) condensation and mass fluxing within the glass melt are prevented.This temperature gradient eliminates the main causes of striae andtherefore reduces compositional variations in the molten glass.

TABLE 2 Glass melting schedule for striae-free As₃₉S₆₁ glass in atwo-zone furnace using the invention in Ref 4. Heating Rate Temperature(° C.) Temperature (° C.) Dwell Step (° C./min) Top Zone Bottom Zone(hours) Furnace Position 1 3 850 750 1 Horizontal 0° fixed 2 — 850 75010 Rocking at ±45° inclination 3 −1 800 700 24 Vertical 90° fixed 4 −0.6360 260 12 Vertical 90° fixed 5 Water quench

This method, which was demonstrated with arsenic-sulfide binary glasses,does not work with all multicomponent glasses, an example of which isGe_(x)As_(y)S_((100-x-y-z))Se_(z), glass due to the potential formationof intermediate phases upon cooling, including crystals, nuclei, andphase separated immiscible glasses, due to slow cooling. FIGS. 5A and 5Bshow a Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glass, 0≤x≤10, 0≤z≤10 and30≤x+y≤45 that was fabricated using this method; crystal precipitatescan be seen above and in the glass. FIG. 5A shows an ampoule 70containing the Ge_(x)As_(y)S_((100-x-y-z))Se glass 71 made by thismethod and crystals 72 above the glass. FIG. 5B is an enlargement ofarea 73.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides high optical quality multicomponent chalcogenide glasseswithout refractive index perturbations due to striae, phase separationor crystal formation using a two-zone furnace and multiple fining steps.The top and bottom zones are initially heated to the same temperature,and then a temperature gradient of about 100° C. is created between thetop zone and the bottom zone. The fining and cooling phase is dividedinto multiple steps with multiple temperature holds.

The present invention enables synthesis of homogeneous, optical qualityglasses for some glass compositions that are not possible using methodsof the prior art. The chalcogenide glasses and fibers described herein,and more specifically glasses and fibers containing primarily arsenic,sulfur, selenium, tellurium, germanium with dopants including antimony,gallium aluminum, indium, bismuth, tin, iodine, bromine, chlorine,fluorine, lanthanum and other elements up to about 10% at each, may besynthesized according to the method of the present invention in formssuitable for optical quality fibers and geometric optics includingwindows, lenses and other devices.

The process of the present invention has the following advantages overthe process of the prior art:

-   -   Fast cooling rate and shorter overall processing time prevent        formation of intermediate phases, including precipitated        crystals, during cooling of the glass melt.    -   Thermal convection heat loss, convection current and mass flux        are eliminated within the bulk molten glass by setting the        temperature of the top zone approximately 100° C. higher than        the bottom zone through all steps of the cooling process.    -   Controlled cooling with multiple temperature hold steps allow        the entire volume of the glass melt to reach the same        temperature, or thereabouts, and prevents the formation of        convection currents within the glass melt during the entire        cooling phase of the process. This contributes to a striae-free,        lower energy, and stable state of the glass melt just before        quenching.    -   Uniform composition and the absence of crystals, nuclei, and        striae in the bulk glass eliminate refractive index        perturbations enabling glass with higher optical quality for        high-performance IR fibers and refractive optical elements.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic overview of a prior art process to synthesizechalcogenide glasses by melt processing.

FIG. 2 is a schematic overview of a rocking furnace in vertical) (90°fixed position of a prior art process.

FIG. 3A is a schematic diagram of thermal convection current in theGe_(x)As_(y)S_((100-x-y-z)) Se_(z) glass melt and glass condensationdrops on top of the cooler ampoule inside the furnace of a prior artprocess. FIG. 3B is a photo of said ampoule after quenching with glasscondensation above the solid glass.

FIG. 4A is an IR-image of a hot steel grating viewed through a 1 inchdiameter, 4.0 inches thick cylinder (both faces polished) ofGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass showing striae in the bulkglass. FIG. 4B is an IR-image of a human hand and fingers viewed througha 1 inch diameter, 4.0 inches thick cylinder (both faces polished) ofGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass showing striae in the bulkglass.

FIG. 5A shows an ampoule containing a Ge_(x)As_(y)S_((100-x-y-z))Seglass made by a prior art process, where 0≤x≤10, 0≤z≤10 and 30≤x+y≤45,showing crystals above the glass. FIG. 5B shows an enlargement of area73 from FIG. 5A.

FIG. 6 is a schematic overview of a furnace used in the currentinvention to synthesize chalcogenide glasses by melt processing.

FIG. 7 is a schematic overview of a rocking furnace in vertical (90°)fixed position such that the axis of the ampoule is vertical.

FIG. 8A is a photo of an ampoule with no glass condensation above theglass. FIG. 8B is an IR-image of a hot steel grating viewed through a 1inch diameter, 4.0 inches thick Ge_(x)As_(y)S_((100-x-y-z)) Se_(z) glasscylinder with both faces polished showing no striae in the uniform bulkglass. FIG. 8C is an IR-image of a human hand and fingers viewed througha 1 inch diameter, 4.0 inches thick Ge_(x)As_(y)S_((100-x-y-z))Se_(z)glass cylinder with both faces polished showing no striae in the uniformbulk glass.

FIG. 9A is an IR image of human hand and fingers viewed through aGe_(x)As_(y)S_((100-x-y-z)) Se_(z) glass cylinder produced from theprocess of the present invention. FIG. 9B is an IR image of human handand fingers viewed through a Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glasscylinder produced from a prior art method revealing refractive indexperturbations in the art glass of the prior art.

FIG. 10A is a photo of a 3.0 inches thick glassGe_(x)As_(y)Se_(z)Te_((100-x-y-z)) ingot (83). FIG. 10B shows anIR-image of a hot steel grating viewed through a 55 mm diameter, 3.0inches thick Ge_(x)As_(y)Se_(z)Te_((100-x-y-z)) glass cylinder with bothfaces polished showing no striae in the uniform bulk glass. FIG. 10Cshows an IR-image of a human hand and fingers viewed through a 55 mmdiameter, 3.0 inches thick Ge_(x)As_(y)Se_(z)Te_((100-x-y-z)) glasscylinder with both faces polished showing no striae in the uniform bulkglass.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a new method to synthesize striae-freechalcogenide glass using melt processing. High optical qualitymulticomponent chalcogenide glasses without refractive indexperturbations due to striae, phase separation or crystal formation aresynthesized using a two-zone furnace and multiple fining steps. The topand bottom zones are initially heated to the same temperature, and thena temperature gradient of about 100° C. is created between the top zone(750° C.) and the bottom zone (650° C.). The fining and cooling phase isdivided into multiple steps with multiple temperature holds. The glassmelt is fined for 3 hours at high temperature—above the temperature atwhich crystal precipitation is known to begin—and then rapidly cooled toa lower temperature below the temperature at which crystal precipitationoccurs where it is held for 3 hours. The glass melt is then cooled andheld at another lower temperature for 3 more hours.

The method of the present invention to synthesize striae-freechalcogenide glass using melt processing is described herein by exampleusing Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glass.

EXAMPLE 3 Process of the Present Invention to Make Striae-FreeGe_(x)As_(y)S_((100-x-y-z))Se_(z) and Other Multicomponent ChalcogenideGlasses

Germanium, arsenic, sulfur, and selenium precursors sufficient toconstitute a glass with the composition of x % at. Ge, y % at. As, z %at. Se and (100-x-y-z) % at. S, where (0≤x≤10, 0≤z≤10 and 30≤x+y≤45) or(0.5≤x≤10, 0.5≤z≤10 and 30≤x+y≤45) are loaded in a silica ampoule underan inert nitrogen gas atmosphere. FIG. 6 shows a schematic overview ofthe furnace used in the present invention to synthesize chalcogenideglasses by melt processing comprising a sealed quartz ampoule 200containing melted Ge_(x)As_(y)S_((100-x-y-z))Se_(z) precursors 203inside a rocking furnace 30 having two independently controllableheaters 31, 32 with a ±45° inclination angle 101. The ampoule 200 iscylindrical in shape and has an axis 201 and a diameter 202, and is thenconnected to a vacuum pump and evacuated for 4 hours at about 1×10⁻⁵Torr. The ampoule has a length, parallel to its axis, and a diameter,perpendicular to the axis, such that the length is greater than thediameter, but the ampoule is not limited to this geometry and may have adiameter greater than its length, as is useful for casting largediameter glass for large optics. Furthermore the ampoule may haveanother shape similar to the shape of a glass product such as a lens orlens preform used in precision lens molding processes. The ampoule isthen sealed using a methane (or hydrogen) /oxygen torch and placedinside a rocking furnace with a ±45° angle of inclination and twoindependently controllable temperature zones where it is heated androcked according to a glass melting schedule, an example of which isshown in Table 3. The bottom of the ampoule is placed at the center ofthe bottom zone where the glass melt is entirely within the bottom zone.

In Step 1, the top and bottom zones of the furnace are heated at a rateof 3° C./min from 20° C. (room temperature) to 680° C. (top) and 680° C.(bottom). In Step 2, the temperatures of the top zone (680° C.) andbottom zone (680° C.) are held constant for 15 hours while the furnaceis rocked at an inclination angle of ±45° to facilitate mixing andhomogenization of the elemental components. In Step 3, the top zonetemperature is increased at a rate of 0.6° C./min to 750° C. and thebottom zone temperature is decreased at a rate of 0.6° C./min to 650° C.while the furnace is rocked at an inclination angle of ±45° to establishthe temperature gradient of 100° C. between the top and bottom zones. InStep 4, the furnace motion is stopped and the furnace was set to avertical position (90° fixed angle). This furnace position andtemperature profile were held for 3 hours to facilitate fining andsettling of the glass melt. In Step 5, the temperatures of the top zoneand the bottom zone are reduced at a rate of 10.0° C./min to 650° C.(top) and 550° C. (bottom) and held for 3 hours. In Step 6, thetemperatures of the top zone and the bottom zone are reduced at a rateof 10.0° C./min to 550° C. (top) and 450° C. (bottom) and held for 3hours. In Step 7, the temperatures of the top zone and the bottom zoneare reduced at a rate of 10.0° C./min to 450° C. (top) and 350° C.(bottom) and held for 0.5 hours. In Step 8, the hot ampoule is removedfrom the furnace, submerged in a room temperature water bath for 9seconds to quench the glass, and is placed in another furnace at 195° C.for 10 hours to anneal the solid glass.

TABLE 3 Glass melting schedule for a Ge_(x)As_(y)S_((100−x−y−z))Se_(z)glass composition in a two-zone furnace using the present invention.Heating Rate Temperature (° C.) Temperature (° C.) Dwell Step (° C./min)Top Zone Bottom Zone (hours) Furnace Position 1 3 680 680 1 Horizontal0° fixed 2 — 680 680 15 Rocking at ±45° inclination 3 +0.6(T), −0.6(B)750 650 1 Rocking at ±45° inclination 4 — 750 650 3 Vertical 90° fixed.Fining. 5 −10.0 650 550 3 Vertical 90° fixed. Fining. 6 −10.0 550 450 3Vertical 90° fixed. Fining. 7 −10.0 450 350 0.5 Vertical 90° fixed. 8Water quench

In general, the steps in the present invention are similar to steps inthe prior art Example 2 to make striae-free 2-component chalcogenideglass (As₃₉S₆₁), but the differences described herein enable thefabrication of multicomponent IR-transmitting chalcogenide glasses withimproved uniformity and without striae.

Step 1 in the present invention allows for an initial melting ofprecursor materials prior to rocking for homogenization and reduces thepotential of abrasion of the ampoule by solid precursors during the nextstep. This differs from Step 1 in the prior art of Example 2 in thatboth furnace zones are set to the same temperature in order to preventdistillation of chemicals from 1 zone to the other via sublimation andcondensation.

Steps 2 & 3 encourage mixing and homogenization of the melted materialduring rocking. In Step 3 of the present invention, a temperaturegradient of 100° C. between the top zone (750° C.) and bottom zone (650°C.) is established while the furnace is rocking. This is done in thelast hour of the rocking phase so that the gradient is established priorto vertical fining in the next steps. The 100° C. temperature gradientis held throughout the remaining process steps 3-7 in order to 1) reduceconvection currents within the glass melt and 2) prevent condensationand mass fluxing within the glass melt. Both phenomena contribute toinhomogeneity and striae in the final glass and are reduced in thisinvention. This differs from the prior art in Example 2, whichestablishes and maintains a temperature gradient throughout the entirerocking phase of the process and may contribute to inhomogeneity inmulticomponent glass melts.

Steps 4-7 differ from the prior art method in Example 2, in that thefining and cooling phase is divided into multiple steps with multipletemperature holds. In Steps 4-7, the ampoule containing the glass meltis positioned such that the glass melt is largely confined within thebottom zone of the furnace and it is being fined for 3 hours first athigh temperature (Step 4) above the temperature at which crystalprecipitation (or another phase separation process) is known to beginand then rapidly cooled to a lower temperature safely below (generally20-200° C. below) the temperature at which crystal precipitation/phaseseparation occurs where it is held for 3 hours (Step 5) to allow for thevolume of glass to reach the same temperature or thereabouts. The glassmelt is then cooled and held at another lower temperature for 3 hours(Step 6) to allow for the volume of glass to reach the same temperatureor thereabouts and then cooled to the quenching temperature (Step 7).These multiple fining steps (3-7) and a consistent 100° C. highertemperature in the top zone prevent thermal convection within the glassduring cooling which allows the uniform conditions in the molten glassestablished in the previous steps to remain and prevents thereincorporation of surface glass into the bulk glass as in the case ofthe prior art Examples 1 and 2. The fast cooling rate (10° C./min) andshorter overall processing time prevent formation of intermediate phasesincluding precipitated crystals and crystal nuclei during cooling of theglass melt.

FIG. 7 shows the measuring points 401, 402, 403, 404, 405 in the furnaceduring the dwell portion of Step 7 in this example. The short dwellsteps allow the entire volume of the glass melt to reach the sametemperature, or thereabouts, prior to the next cooling step. Thetemperature was measured various points 401, 402, 403, 404, 405 alongthe length of the quartz ampoule 200 containingGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass melt 203. The heater 30 of thetop zone 33 was set to 450° C. and the heater 32 of the bottom zone 34was set to 350° C. in this example. The measured temperatures were asfollows: T₄₀₁=451° C., T₄₀₂=450° C., T₄₀₃=352° C., T₄₀₄=351° C., andT₄₀₅=350° C.

During water quenching of Step 8, the viscosity of the glass increasesrapidly as the glass melt cools but thermal stresses are less than thosein the method of the prior art due to shorter quench time in the presentinvention. FIG. 8A shows a photo of a Ge_(x)As_(y)S_((100-x-y-z))Se_(z)glass of the present invention inside an ampoule 201 with no glasscondensation or crystal nuclei above the glass melt 81. FIG. 8B shows anIR-image of a hot steel grating 52 viewed through a 1 inch diameter, 4.0inches thick Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glass cylinder 82 withboth faces polished showing no striae in the uniform bulk glass. FIG. 8Cshows an IR-image of a human hand and fingers 62 viewed through a 1 inchdiameter, 4.0 inches thick cylinder 82 (both faces polished) of aGe_(x)As_(y)S_((100-x-y-z))Se_(z) glass with no detectable striae,crystallites or refractive index perturbations in the bulk glass.

The process of the present invention produces high optical qualitymulticomponent chalcogenide glasses without refractive indexperturbations due to striae, phase separation or crystal formation.Comparing the photographs of the glasses inside the ampoules for theglass made using the prior art method (FIG. 5A) and the glass made usingthe present invention (FIG. 8A), it is evident that the formation ofcrystals has been reduced by the present invention. Furthermore,comparing the IR images of the glass prepared using the presentinvention (FIGS. 8B and 8C) with those of the glass prepared using theprior art methods (FIGS. 4A and 4B) reveals a dramatic improvement inoptical quality and homogeneity for these glasses. This is demonstratedin FIGS. 9A and 9B, where the Ge_(x)As_(y)S_((100-x-y-z))Se_(z) glass 82produced from the process of the present invention (FIG. 9A) is clearwhen viewed with an infrared camera, while the glass 50 of the prior art(FIG. 9B) contains many refractive index perturbations 61.

EXAMPLE 4 Process of the Present Invention to Make Striae-FreeGe_(x)As_(y)Te_((100-x-y-z))Se_(z) and Other Multicomponent ChalcogenideGlasses

In this example, glass containing germanium, arsenic, selenium, andtellurium is fabricated without striae and without crystalliteinclusions using the process described in Example 3 above with the maindifference being the precursor elements and their quantities and thedetails of the heating schedule. Germanium, arsenic, selenium andtellurium precursors sufficient to constitute a glass with thecomposition of x % at. Ge, y % at. As, z % at. Se, and (100-x-y-z) % at.Te, where (0≤x≤25, 30≤x+y≤55, and 0≤z≤20) or (0.5≤x≤25, 30≤x+y≤55, and0.5≤z≤20) are loaded in a silica ampoule under an inert nitrogen gasatmosphere. The ampoule is then sealed using a methane/oxygen orhydrogen/oxygen torch and placed inside a rocking furnace with a ±45°angle of inclination and two independently controllable temperaturezones where it is heated and rocked according to a glass meltingschedule, an example of which is shown in Table 4 and described in moredetail below. The bottom of the ampoule is placed at the center of thebottom zone such that the glass melt is entirely within the bottom zone.

In general, the steps in this example correspond to steps in the Example3 but with different starting precursors and the times and temperaturesused in the heating schedule.

TABLE 4 Glass melting schedule for a Ge_(x)As_(y)Se_(z)Te_((100−x−y−z))glass composition in a two-zone furnace using the present inventionHeating Rate Temperature (° C.) Temperature (° C.) Dwell Step (° C./min)Top Zone Bottom Zone (hours) Furnace Position 1 3 650 650 1 Horizontal0° fixed 2 — 650 650 15 Rocking at ±45° inclination 3 +0.6(T), −0.6(B)750 650 1 Rocking at ±45° inclination 4 — 750 650 3 Vertical 90° fixed.Fining. 5 −10.0 650 550 3 Vertical 90° fixed. Fining. 6 −10.0 550 450 3Vertical 90° fixed. Fining. 7 −10.0 300 200 0.5 Vertical 90° fixed. 8Water quench

In Step 1, the top and bottom zones of the furnace are heated at a rateof 3° C./min from 20° C. (room temperature) to 650° C. (top) and 650° C.(bottom). In Step 2, the temperatures of the top zone (650° C.) andbottom zone (650° C.) are held constant for 15 hours while the furnaceis rocked at an inclination angle of ±45° to facilitate mixing andhomogenization of the elemental components. In Step 3, the top zonetemperature is increased at a rate of 0.6° C./min to 750° C. and thebottom zone temperature is remained at 650° C. while the furnace isrocked at an inclination angle of ±45° to establish the temperaturegradient of 100° C. between the top and bottom zones. In Step 4, thefurnace motion is stopped and the furnace was set to a vertical position(90° fixed angle). This furnace position and temperature profile wereheld for 3 hours to facilitate fining and settling of the glass melt. InStep 5, the temperatures of the top zone and the bottom zone are reducedat a rate of 10.0° C./min to 650° C. (top) and 550° C. (bottom) and heldfor 3 hours. In Step 6, the temperatures of the top zone and the bottomzone are reduced at a rate of 10.0° C./min to 550° C. (top) and 450° C.(bottom) and held for 3 hours. In Step 7, the temperatures of the topzone and the bottom zone are reduced at a rate of 10.0° C./min to 300°C. (top) and 200° C. (bottom) and held for 0.5 hours. In Step 8, the hotampoule is removed from the furnace, submerged in a room temperaturewater bath for 6 seconds to quench the glass melt forming a solid glassingot. The ampoule containing the solid glass ingot is then placed inanother furnace at 195° C. for 10 hours to anneal the solid glass andreduce stress due to quenching. FIGS. 10A-10C shows the results of thisexample. FIG. 10A shows a photo of a 3.0 inches thick glassGe_(x)As_(y)Se_(z)Te_((100-x-y-z)) ingot 83. FIG. 10B shows an IR-imageof a hot steel grating 52 viewed through the 55 mm diameter, 3.0 inchesthick Ge_(x)As_(y)Se_(z)Te_((100-x-y-z)) glass cylinder 83 with bothfaces polished showing no striae in the uniform bulk glass. FIG. 10Cshows a human hand and fingers 62 viewed through the 55 mm diameter, 3.0inches thick Ge_(x)As_(y)Se_(z)Te_((100-x-y-z)) glass cylinder 83 withboth faces polished showing no striae in the uniform bulk glass.

This invention has been demonstrated usingGe_(x)As_(y)S_((100-x-y-z))Se_(z) and Ge_(x)As_(y)Se_(z)Te_((100-x-y-z))glasses in the above example but can also be applied to othertwo-component and multi-component chalcogenide glasses such as but notlimited to, arsenic, sulfur, selenium and tellurium based glasses andother multi-component chalcogenide and chalcohalide glasses containingantimony, gallium aluminum, indium, bismuth, tin, iodine, bromine,chlorine, fluorine, lanthanum and other elements. The present inventioncould also be applied to the fabrication of other glasses (for examplesilicates, borates, fluorides, phosphates and others) or processing ofviscous liquids (for example polymer melts, metals, salts and otherliquids) where homogeneity is desired.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A striae-free chalcogenide glass made by themethod, comprising: loading germanium, arsenic, sulfur, and seleniumprecursors into an ampoule, wherein the germanium, arsenic, sulfur, andselenium precursors are sufficient to constitute a glass with thecomposition of x % at. germanium, y % at. arsenic, z % at. selenium, and(100-x-y-z) % at. sulfur, wherein 0.5≤x≤10, 30≤x+y≤45, and 0.5≤z≤10,sealing the ampoule, and placing the ampoule in a rocking furnace,wherein the rocking furnace comprises a top zone and a bottom zone,wherein the top and bottom zones are two independently controllabletemperature zones; heating both the top and bottom zones at a rate of 3°C. per minute to a Step 1 temperature, wherein the Step 1 temperature isthe same for both the top and bottom zones; maintaining the Step 1temperature in the top and bottom zones while rocking the furnace at aninclination angle of +45° for 15 hours; increasing the temperature ofthe top zone at a rate of 0.6° C. per minute to a Step 3 top zonetemperature and decreasing the temperature of the bottom zone at a rateof 0.6° C. per minute to a Step 3 bottom zone temperature while rockingthe furnace at an inclination angle of +45°, wherein there is atemperature gradient of 100° C. between the Step 3 top zone temperatureand the Step 3 bottom zone temperature; setting the furnace to avertical position and maintaining the Step 3 top zone temperature andthe Step 3 bottom zone temperature for 3 hours; decreasing thetemperature of the top zone at a rate of 10° C. per minute to a Step 5top zone temperature, decreasing the temperature of the bottom zone at arate of 10° C. per minute to a Step 5 bottom zone temperature, andmaintaining the Step 5 top zone temperature and Step 5 bottom zonetemperature for 3 hours, wherein the Step 5 top zone temperature is 100°C. lower than the Step 3 top zone temperature, and wherein the Step 5bottom zone temperature is 100° C. lower than the Step 3 bottom zonetemperature; decreasing the temperature of the top zone at a rate of 10°C. per minute to a Step 6 top zone temperature, decreasing thetemperature of the bottom zone at a rate of 10° C. per minute to a Step6 bottom zone temperature, and maintaining the Step 6 top zonetemperature and Step 6 bottom zone temperature for 3 hours, wherein theStep 6 top zone temperature is 100° C. lower than the Step 5 top zonetemperature, and wherein the Step 6 bottom zone temperature is 100° C.lower than the Step 5 bottom zone temperature; decreasing thetemperature of the top zone at a rate of 10° C. per minute to a Step 7top zone temperature, decreasing the temperature of the bottom zone at arate of 10° C. per minute to a Step 7 bottom zone temperature, andmaintaining the Step 7 top zone temperature and Step 7 bottom zonetemperature for 30 minutes, wherein the Step 7 top zone temperature isat least 100° C. lower than the Step 6 top zone temperature, and whereinthe Step 7 bottom zone temperature is at least 100° C. lower than theStep 6 bottom zone temperature; and removing the ampoule from thefurnace and water quenching the glass.